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Identification of an iridium-containing compound with a formal oxidation state of IX


One of the most important classifications in chemistry and within the periodic table is the concept of formal oxidation states1,2,3,4. The preparation and characterization of compounds containing elements with unusual oxidation states is of great interest to chemists5. The highest experimentally known formal oxidation state of any chemical element is at present VIII2,3,4, although higher oxidation states have been postulated6,7. Compounds with oxidation state VIII include several xenon compounds8 (for example XeO4 and XeO3F2) and the well-characterized species RuO4 and OsO4 (refs 2, 3, 4). Iridium, which has nine valence electrons, is predicted to have the greatest chance of being oxidized beyond the VIII oxidation state1. In recent matrix-isolation experiments, the IrO4 molecule was characterized as an isolated molecule in rare-gas matrices9. The valence electron configuration of iridium in IrO4 is 5d1, with a formal oxidation state of VIII. Removal of the remaining d electron from IrO4 would lead to the iridium tetroxide cation ([IrO4]+), which was recently predicted to be stable10 and in which iridium is in a formal oxidation state of IX. There has been some speculation about the formation of [IrO4]+ species11,12, but these experimental observations have not been structurally confirmed. Here we report the formation of [IrO4]+ and its identification by infrared photodissociation spectroscopy. Quantum-chemical calculations were carried out at the highest level of theory that is available today, and predict that the iridium tetroxide cation, with a Td-symmetrical structure and a d0 electron configuration, is the most stable of all possible [IrO4]+ isomers.

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Figure 1: Mass spectra of the iridium oxide cations.
Figure 2: Infrared photodissociation spectra of the [193IrO4]+·Arn (n = 1–4) cations.
Figure 3: Optimized structures and energetic ordering of the different [IrO4]+ isomers.

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This work was supported by the Ministry of Science and Technology of China (2013CB834603 and 2012YQ220113-3), the National Natural Science Foundation of China (grant nos 21173053, 21433005 and 91026003), the Committee of Science and Technology of Shanghai (13XD1400800), the Fonds der Chemischen Industrie and the GRK 1582 ‘Fluorine as a key element’. We also acknowledge the Natural Sciences and Engineering Research Council of Canada for a Discovery Grant (G.J.S.) and for a postgraduate scholarship (J.T.G.). We are grateful to I. Krossing and H. Hillebrecht for their support.

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Authors and Affiliations



G.W. and M.Z. designed and performed the gas-phase experiments, J.T.G. and G.J.S. attempted to synthesize [IrO4] salts, J.S., J.L., T.S. and S.R. performed the quantum chemical calculations. M.Z., G.J.S., J.L. and S.R. wrote the paper and supervised the experimental and theoretical parts. All authors discussed the results and commented on the manuscript at all stages.

Corresponding authors

Correspondence to Mingfei Zhou, Gary J. Schrobilgen or Sebastian Riedel.

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The authors declare no competing financial interests.

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Wang, G., Zhou, M., Goettel, J. et al. Identification of an iridium-containing compound with a formal oxidation state of IX. Nature 514, 475–477 (2014).

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